Such an apparatus is described in EP 0 355 168 A1. It comprises a roller which is supported for rotation on a pivotable lever and around which a tensioned wire electrode is passed. The lever rests on a pressure sensor. A brake disposed upstream in the direction of wire movement is controlled in response to the tensile stress of the wire electrode as detected by the pressure sensor so that the tension in the wire electrode will adopt a predetermined value.
German Patent DE 26 25 349 C2 discloses an apparatus for machining workpieces by spark erosion or electrochemically, using a wire or ribbon-type electrode, a separately driven pair of drive rollers for supplying or removing the electrode being arranged at the two sides of the working length located between guide members. Control of the electrode tension is effected by means of a pivotable roller which is loaded by the electrode against spring pressure or by gravity. A sensor detects the degree of deflection of the roller. The rotary speed of a motor which tensions the electrode is derived from the sensor output signal so that the deflection of the roller is regulated in such a way that the roller will maintain a certain position as much as possible. The means which controls the tensile force is located between the two pairs of drive rollers which are driven at invariable torques. The rotary speed of the drive motor for the drive roller pair is controlled in response to the deflection of the roller.
Another device for controlling the tension in a wire electrode of an electric discharge machine is known from German patent publication DE 32 08 036 A1. This device serves to control the tensile stress between a supply-side drive roller pair toward an operating range and a wire supply reel which is driven by a motor to pay off wire. A roller which is supported for rotation on a pivotable lever and around which the wire is passed is arranged in the wire path between the supply reel and the supply-side drive roller pair. The lever is spring loaded with a view to tensioning the wire electrode. A sensor means is disposed at one end of the lever to detect the deflection of the lever and thus the magnitude of the tensile stress and to generate a control signal by which the motor driving the supply reel is regulated.
DE 22 65 708 C1 describes a means used, in combination with an apparatus for the removal of a wire or ribbon-type electrode in machines for processing workpieces by spark erosion or electrochemically, to cause unvarying tension of the electrode at the discharge end of the electric discharge machine. To accomplish this, a deflectable pulley is provided which causes a loop to be formed of the electrode between a drive roller pair at the discharge end and the take-up reel which is driven by a variable speed motor. The degree of deflection from a central position serves for control of the wind-up speed of the reel such that a constant tension of the electrode is provided at the discharge-side drive roller pair.